Method and apparatus for applying voltage in electrophotography

ABSTRACT

In an electrophotographic operation, a method and apparatus for applying voltage continuously without accumulating an electric charge in a surface layer of a dielectric resulted from applying voltage thereon while sandwiching a sensitive paper or plate or a transcription paper by the dielectric and an earth member.

United States Patent Inventors Appl. No.

Filed Patented Assignee Priorities Takuhei Kimura; Masahiro Takeda, both of Tokyo, Japan 808,132

Mar. 18, 1969 Dec. 7, 197 1 Iwatsu Electric Co., Ltd.

Tokyo, Japan Mar. 19, 1968 Japan July 4, 1968, Japan, No. 43/46742; Aug. 9, 1968, Japan, No. 43168379 METHOD AND APPARATUS FOR APPLYING VOLTAGE 1N ELECTROPHOTOGRAPl-IY 20 Claims, 15 Drawing Figs.

US. Cl

317/3 Int. Cl 1101b l/00 Field of Search 317/3-4,

[56] References Cited UNITED STATES PATENTS 3, 1 72,024 3/1965 Gundlach 317/262 3,195,362 7/1965 Stuetzer 310/5 X 3,521,126 7/1970 Granzow et a1.. 317/262 X 2,980,834 4/1961 Tregay 317/262 Primary Examiner-D. F. Duggan Assistant Examiner-Ulysses Weldon Attorneys- Robert E. Burns and Emmanuel J. Lobato ABSTRACT: In an electrophotographic operation, a method and apparatus for applying voltage continuously without accumulating an electric charge in a surface layer of a dielectric resulted from applying voltage thereon while sandwiching a sensitive paper or plate or a transcription paper by the dielectric and an earth member.

PATENTED DEC 7 19" sum 1 or 3 Q" {PRIOR ART PATENTEUDEC nan 3526.260

SHEET 2 BF 3 Fig. 7A Fig. 75

METHOD AND APPARATUS FOR APPLYING VOLTAGE IN ELECTROPHOTOGRAPHY The present invention relates to a method and apparatus for applying voltage between a dielectric and an electroconductive member in electrophotography, more particularlyrelates to amethod and apparatus for applying voltage between a dielectric and an electroconductive member to charge static electricity upon a sensitive paper or plate, or to transfer toner from a sensitive paper provided with a developed image to a paper, while preventing accumulation of, electric charge in a surface layer of the dielectric so that continuous operation of electrophotography can be carried out. The above-mentioned electric charge accumulated in the surface layer of the dielectric is referred to-hereinafter asfspace charge."

Generally, an electrostatic charging system utilizing fine metallic wires or needles is employed in most of the charging devices utilized for conventional electrophotographic apparatus. In this system, a: high DC voltage is applied to the fine metallic wires or needles so that a sensitive layer of photoconductive material of a copy paper such as a paper coated with zinc oxide or selenium sensitive plate can be charged by a corona discharge from the metallic wire or needle without contacting any elements of the charging device. However, in spite of having the above-mentioned advantage, the charging speed is not practically high, therefore the slow charging speed may be considered as a drawback of this system. In a well-known voltage applying device provided with a voltage applying roller peripherally covered by a dielectric, a DC voltage is applied to the voltage applying roller while a sensitive paper contacts the roller, an electric charge is accumulated in a surface layer of the dielectric, consequently, the potential of the accumulated electric charge, that is, the potential of the space charge is increased so that the voltage applying operation between the dielectric and the electroconductive member becomes impossible to carrying out continuously, in other words, the electrostatic charging upon the sensitive paper is stopped. To avoid the above-mentioned trouble, if a DC voltage, which is sufficiently high enough to overcome the electric charge in the mentioned surface layer of the dielectric of the roller, is applied to the voltage applying roller, the dielectric property of the roller is broken down, in other words, the dielectric of the voltage applying roller loses its function as a dielectric. This drawback is observed in the case of the using the above-mentioned voltage applying device utilized for a transfer apparatus in electrophotography.

The principal object of the present invention is to provide a novel, practical method and apparatus for applying voltage between a dielectric and an electrocunductive member in electrophotography, wherein the above-mentioned drawbacks of conventional voltage applying device are eliminated.

Another object of the present invention is to provide a method and apparatus for charging static electricity upon a sheet material in electrophotography, wherein the above-mentioned drawbacks of conventional voltage applying devices are eliminated.

A further object of the present invention is to provide a method and apparatus for transferring toner from a sensitive paper provided with a developed image to a paper, while preventing accumulation of electric charge in a surface layer of the dielectric so that continuous operation of electrophotography can be carried out.

Another object of the present invention is to provide a compact apparatus having a very simple construction so that continuous operation for charging static electricity upon the sheet material can be carried out at a high speed.

Further features and advantages of the invention will be apparent from the ensuing description with reference to the accompanying drawings to which the scope of the invention is in no way limited.

H0. 1 is a schematic side view of a conventional voltage applying apparatus provided with a roller covered by a dielectric showing the characteristic features of the voltage applying apparatus,

FIG. 2 is a schematic side view of a typical embodiment of the voltage applying apparatus according to the present invention,

FIG. 3 is an explanatory drawing showing a principle of the I voltage applying apparatus shown in FIG. 2,

FIGS. 4, 5 and 6 are schematic side views of three embodiments of the voltage .applyingapparatus according to the invention, respectively. I

FIGS. 7A and 7B are explanatory drawings showing a principle of the voltage applying apparatus shown in H6. 6,

FIGS. 8 and 9 are schematic side views of modified embodiments of the voltage applying apparatus of the invention,

FIGS. 10, 11 and 12 are schematic side views of, three embodiments of the voltageapplying apparatus shown in-FlGS. 2, 3 and 6, respectively, utilized for transferring an image in electrophotography of a dry system,

FIGS. 13 and 14 are schematic side views of two embodiments of the voltage applying apparatus shown in FIGS. 4 and 6 respectively, utilized for transferring an image in electrophotography of a wet system- To aid in understanding the present invention, firstly a conventional apparatus for voltage application in electrophotography is illustrated.

Referring to FIG. 1 a conventional. voltage applying apparatus comprisesa voltage applying roller 1 composed of an electroconductive roller la peripherallycovered with a dielectric layer 1b such as polyester urethane rubber. an earth roller 2 disposed under the voltage applying roller 1 so that a paper is passed therethrough, a DC voltage source 3. of above 4 KV being connected to both rollers l and 2. The earth or ground roller 2 is always urged towards the voltageapplying roller 1 and both rollers 1 and 2 are driven to carry the paper. When a high DC voltage, for example DC voltage of 5 KV is applied to the voltage-applying roller 1, and asensitive paper 4 is fed into a nip of the rollers l and 2, a sensitive layer of the paper 4 is charged. Further, the voltage applying roller 1 and the earth roller 2 carry the paper 4. Consequently, there is an advantage in making the voltage applying device compact on the other hand, an electric charge is accumulated in a surface layer of the dielectric lb of the voltage applying roller 1. Consequently, the potential of the space charge is increased so that the voltage applying operation to the paper 4 becomes impossible to carry out continuously, in other words, the electrostatic charging on the paper 4 is stopped in a few minutes. If a sufficiently high DC voltage is applied to overcome the aforementioned trouble,'the dielectric property of the roller l is broken down and loses its function as a dielectric. Therefore, a 'voltage applying apparatus such as the one shown in FIG. 1 may be understood as an impractical apparatus in electrophotography.

From out fundamental study and repeated experiments, a novel and practical method and apparatus for applying voltage between or across a dielectric and an electrocunductive member in electrophotography was developed. This apparatus can be used continuously.

The principle of the present invention is based upon the fact that anelectric charge accumulated in a surface layer of a dielectric by applying a voltage can be eliminated by applying another voltage having reverse polarity to the accumulated charge in the dielectric. There are two ways to eliminate the mentioned electric charge, that is, in the case of a voltage applying roller covered about its peripheral surface by a dielectric and an earth member urged toward the voltage applying roller when the voltage applying roller is charged between the dielectric and the electroconductive roller by a high DC voltage and a sensitive paper or paper is fed to a nip between the roller and the earth member. another voltage having reverse polarity is applied to the voltage applying roller at a part of the peripheral surface exclusive, of the nip and its adjacent position of the roller, thereby the space charge of the dielectric of the roller can be eliminated. Another method is carried out as follows, that is, in the case of a voltage applying member peripherally covered with a dielectric and an earth member urged toward the voltage applying member, when a sensitive paper or transcript paper is sandwiched by these members, a voltage being applied to the voltage applying member, and when the charged paper is delivered from these members, another voltage having reverse polarity is applied to the voltage applying member, thereby the space charge of the dielectric of the voltage applying member can be eliminated. By applying the above-mentioned principle to the voltage applying apparatus in electrophotography, the drawbacks shown in the conventional apparatus can be completely eliminated.

Next, several practical embodiments of the voltage applying apparatus according to the invention are hereinafter illustrated.

Referring to FIG. 2, showing an embodiment of the voltage applying apparatus of the invention, a voltage applying roller 5 is composed of an electroconductive roller portion 5a peripherally covered by a dielectric layer 5b. An earth roller 6 is disposed under the voltage applying roller 5 so that a sensitive paper or photoconductive sheet 7 is sandwiched by these rollers 5 and 6 and carried therethrough. The roller 5 is cyclically driven by a driving means (not shown). An electroconductive roller 9 is rotatably disposed upon the voltage applying roller 5 so that the roller 9 is driven by a frictional contact with the roller 5. A DC voltage source 8 is connected to both rollers 5 and 6 while another DC voltage source is connected to the voltage source 8 and the roller 9.

Further, referring to FIG. 3, when a high DC voltage, for instance, a voltage in a range from 3,000 V to 15,000 V, is applied to the voltage applying roller 5 during the operation, a sensitive layer 7a of the paper 7 can be charged at a contacting position A of the cyclically driven roller 5 with the paper 7. Further an electric charge is accumulated in a portion of the exterior surface layer of the dielectric 5a. However, as the voltage applying roller 5 is charged by the electroconductive roller 9 at the opposite position B of the roller 5 with respect to the center of the roller 5, the space charge accumulated in the dielectric 5b is eliminated at the above-mentioned position B of the voltage applying roller 5 where another voltage having reverse polarity is applied. In other words, as the roller 5 rotationally moves through its cycle of operation, the surface charge accumulated at location A is eliminated at location B. As mentioned above, the additional voltage applying operation is carried out continuously to the voltage applying roller 5 so that the accumulation of the space charge in the dielectric 5b of the roller 5 can be satisfactorily prevented. Therefore, the voltage applying operation to sensitive papers can be continuously carried out for a long time when the continuous operation of electrophotography is required.

Further, as the earth roller 6 is always urged toward the voltage applying roller 5 with a suitable contact pressure so that a sensitive paper 7 can be carried with the rotation of both rollers 5 and 6, an auxiliary device for feeding or delivering of the paper 7 is not required, therefore, the above-mentioned voltage applying apparatus can be made into a very compact and simple construction.

In the voltage applying apparatus shown in FIG. 4 which is a modified embodiment shown in FIG. 2 of the invention, a DC voltage is only applied between the electroconductive roller 9 and the earth roller 6 so that a similar effect as to the embodiment shown in FIG. 2 can be attained.

In the voltage applying apparatus shown in FIG. 5, an electroconductive piece such as a metallic piece 12 is used for applying voltage having the reverse polarity to the voltage applying roller 5 at a position which differs from the contact position of the roller 5 with a sensitive paper 13 instead of the electroconductive roller 9, and an earth plate I1 is used instead of the earth roller 6 in the embodiment shown in FIG. 2. A DC voltage source 8 is connected to the roller 5 and another DC voltage source 10 is connected to the metallic piece 12 and the DC voltage source 8. The metallic piece 12 always contacts with the voltage applying roller 5. Therefore, when the sensitive paper 13 is fed into the nip of the roller 5 and the earth plate 11 a sensitive layer 130 of the paper 13 is charged.

In the above-mentioned embodiment shown in FIG. 5, the accumulation of the space charge in the dielectric 5a of the roller 5 is prevented in a similar way as that of the voltage applying apparatus shown in FIG. 2.

The voltage applying apparatus shown in FIG. 6 is a modified embodiment of the present invention, that is, an AC voltage is applied to the voltage applying roller 5 in such a way that the polarity of the voltage applied to the roller 5 during sandwiching a paper by the roller 5 and the earth roller 6 is reversed with respect to that of the voltage applied to the roller 5 during an absence of a paper between both rollers 5 and 6, and the above-mentioned alternative voltage applications are carried out synchronously with the feeding of the papers. In the drawing, the voltage applying roller 5 is composed of an electroconductive roller 5a peripherally covered with a dielectric layer 5b. The earth roller 6 is always urged toward the voltage applying roller 6 so that a sensitive paper 14 can be carried by the rotation of both rollers 5 and 6. The sensitive papers 14 are put in a box 15 and fed into a nip of both rollers 5 and 6 by means of a feed roller 16. An AC voltage source 17 is connected to both rollers 5 and 6. As already illustrated, it is necessary that the feed speed of the paper 14 to the rollers 5 and 6 be synchronized with the frequency of the AC voltage source. Therefore, voltage having a suitable polarity to charge the paper 14 is applied during sandwiching the paper 14 by both rollers 5 and 6, while voltage having the reverse polarity is applied to the voltage applying roller 5 during a period after delivering the paper I4 from the nip of both rollers 5 and 6 and before feeding a fresh paper 14 into the nip of the both rollers 5 and 6. A pair of DC voltages having reverse polarities may be alternately applied to the abovementioned voltage applying roller 5 instead of application of the AC voltage.

By the above-mentioned method of alternately applying voltage of different polarities to the voltage applying roller 5, an electric charge accumulated in the dielectric layer 5b of the roller 5 can be satisfactorily eliminated.

Referring to FIGS. 7A, 7B, the above-mentioned function of the voltage applying apparatus is hereinafter illustrated in detail. A voltage having a polarity represented by an arrow (FIG. 7A) is applied between the roller 5 and paper 14 at their contact position C during a period T of sandwiching the paper 14 with both rollers 5 and 6, and a voltage having the reverse polarity represented by another arrow (FIG. 7B) is applied to the contact position D of both rollers 5 and 6 during a period T, of absence of paper 14 between both rollers 5 and 6. The period T is a period between a time after delivery of the paper I4 from the nip of both rollers 5 and 6 and a time before fresh paper 14 is fed into the above-mentioned nip. Therefore, the electric charge accumulated in the dielectric layer 5b of the roller 5 during the period T is eliminated by applying a voltage having the reverse polarity, to the roller 5 for the period T This alternate voltage application is satisfactorily carried out without trouble for a long time. Further, the voltage applying roller 5 and the earth roller 6 work as a feeding and delivering device of the paper 14. Consequently, auxiliary feeding and delivering devices are not required for the voltage applying apparatus shown in FIG. 6, besides a feed roller 16 of the box 15. Therefore, the voltage applying apparatus shown in FIG. 6 can be made into a compact and simple construction.

Other modifications of the voltage applying apparatus of the invention are shown in FIGS. 8 and 9. In FIG. 8, the modified voltage applying device comprises a pair of electroconductive rollers 18a and 18b felted by an endless belt made by a dielectric 19, an electroconductive plate 20 disposed under the endless belt 19 in such a way that the plate 20 is always urged toward the endless belt 19, an electroconductive roller 9 disposed upon the endless belt 19, a DC voltage source 8 connected to the roller 18!; and the plate 20 and another DC voltage source 10 connected to the roller 9 and the DC voltage source 8, as shown in the drawing. The endless belt is driven by the rotation of the roller 17 and the roller 9 is driven ,by frictional contact with the endless belt 19 during .the rotation of the endless belt 19.

in FIG. 9, the modified voltage, applying apparatus of the invention comprises a pair of feed rollers 2 and a pair of delivery rollers 23, a voltage applying member 2-1 composed of an electroconductive plate 21a covered by a dielectric 21b and an earth plate 20 always urged toward the dielectric 21b, and AC voltage source .17 is connected to the plates 21a and 20. The voltage applying member 21 is supported by a shifting means (not shown) so as to shift upwards at the time of feeding and,

delivering a sensitive paper 24. The rollers 22 and 23, 23b are intermittently rotated at a synchronized time to the shifting motion of the plate 21. The above-mentioned intermittent rotation of these rollers .22 and 23, and the shifting motion of the plate 21 must be carried out so thatthe voltage having. the

reverse polarities are alternative applied to the plate 21a. ln

the above-mentioned two voltage applying devices shownin FIGS. 8 and 9, the space chargesaccumulated in the dielectric. of the voltage applying. members are satisfactorily eliminated in the same manner as illustrated in the embodiment of the voltage applying apparatus shown in FIGS. 2 and 6.-The last embodiment of the apparatus shown in FlG. 9, is an exception with respect to the compactness of the'apparatus because of, applying the additional feeding and delivering devices of the paper and also the shifting means. a

The above-mentioned voltage applying apparatus of the present invention can be satisfactorily used for a transferring apparatus in electrophotography. Two types of applications of. the above-mentioned voltage applying apparatus are hereinafter illustrated.

Referring to FIG. 10, in a voltage applying apparatus for.

utilizing as an element of an electrophotographic transcription apparatus the apparatus has a similar construction to the volt age applying apparatus shown in H0. 2 except that a pair ofv guide plates 25, 26 are additionally disposed at positionsbe fore and after the nip of the voltageapplying roller 5 and the earth roller 6 so that superimposed papers are carried along.

the passage formed by the guide plates 25, 26. In this embodiment, a space charge accumulated in the dielectric 5b duringv the operation can be satisfactorily eliminated. When a paper,

28 superimposed upon a sensitive paper 27 which is already:

provided with a developed image and these superposed papers 27 and 28 are fed into nip of the rollers 5 and.6,if a toner, which forms the image has positive polarity, a DC voltage hav-v ing negative polarityis appliedtothe voltage applying roller-5,

thereby, the toner is transferred to the paper 28 satisfactorily.

while these papers 27 and 28'are carried through both rollers,

5 and 6. As mentioned above, there are no troubles caused bythe space charge in the dielectric Sbof the voltage applying trophotography of a dry system comprises similar elements .to

that shown in FIG. 10, besides the electroconductive roller 9-is omitted and an applying AC voltage source 17 is connected to the voltage applying roller 5 and the earth roller 6. The principle of preventing the accumulation of the space charge inthe dielectric 5b f the roller 5 is the same as that of the voltage applying apparatus shown in FIG. 6. In the both above-mentioned embodiments shown in FIGS. 11 and I2, the similar results of transcription to that of the embodiment shown in FIG. 10 can be obtained. I

Embodiments of the voltage applying apparatus used for transcription in the electrophotography of a wet system are shown in F I65. 13 and 14. in these embodiments the apparatus comprises the same elements as that of the embodimentsshown in FlGS. 10, 1'1 and 12, respectively, except that particular elements for-a wet system are.used instead of the guide. plates 25 and 26 The auxiliaryelements for a wet system comprise a bath 29 for containing a transfer agent 30. which has the ability to weaken the function of a fixer forfixing the developed image. on a-sensitive paper without injuring a sensitive layer of a sensitive paper produced by an elec-. trophotographic apparatus, and a curved guide plate 31 disposed partly in the transfer agent 30 as shown in the drawing. Therefore, in both the above-mentioned apparatus, when a sensitive .paper 32 is fed into the bath 29 and advanced towards the voltage applying apparatuswhile passing under the curvedguide plate 3L a paper 33 is directly fed to the voltage applying apparatus being superimposed upon the sensitive paper 32, if a toner fixedonthe sensitive paper 32 is charged in positive polarity and the paper 33-is. charged in negative polarityby. the voltage ,applyingroller 5, the image made by the toneris transferred-from the sensitive paper 32 to the paper 33 without any deformation. inthe above-mentioned five embodiments,the voltage applying roller 5 and the earth roller flare rotatedtowarddirections represented by arrows, respectively, without any additional conveying device for the papers..

- As is mentioned above, even when there is no paper betweenthe voltage applying roller,and. the earth roller.(or earth plate), no formationofa short circuit between the rollers takes place. Consequently, there is noproblem in the system of the present invention, with respect to thesize of the paper. Qn thecontrary, severalkinds of voltageapplying rollers and earth rollers .havingdifferent sizes are required to be kept in reserve for the conventional voltage applying device of the transfer apparatus in .electrophotography. Further, as clearlyillustrated in the. embodiments of, the voltageapplying apparatus of the invention shownin FIGS. 10 and 12 there are no troubles caused by the electric charge accumulated in the dielectric Sbof thevoltage applying roller5 to carry out the operation.

Whilethe invention has been described in conjunction with certain embodiments thereof, it is to be understood that various modifications and changes may be made without departing from the spirit and scope of the invention.

What is claimed is; i

l. A method for charging staticelectricity upon a sheet material inv electrophotography comprising: sandwiching said sheet material between a rotationally driven dielectric roller andan electroconductive member. urged toward said rollers; applying a voltage between said dielectric roller andsaid electroconductive .member effective to impart a static charge upon saicl sheetmaterial while preventing-accumulation of spacecharge, in. a surface layer of saiddielectric roller, and said prevention of space charge accumulation in a surface layer of said dielectric roller is carried out by applying another voltage having a reversepolarity with respectto said first- I said dielectric member and said electrical conductor; applying h I a voltage across said dielectric member and said electrical v conductor whensaid photoconductive sheet is in said sheet passageway thruslytdefining one part of said cycle of sufficient magnitude to electrostatially charge said photoconductive sheet accompanied byaccumulation of an electrostatic space charge in an exterior surface portion of said dielectric member; and then applying a voltage across said dielectric member when same is atanother part of its cycle. different than said one part effective to eliminate the electrostatic space charge accumulated in said exterior surface portion thereof.

3. A method according to claim 2, wherein said step of applying a voltage across said dielectric member and said electrical conductor comprises applying a DC voltage thereacross to electrostatially charge said photoconductive sheet accompanied by building up of an electrostatic space charge of one polarity; and wherein said step of applying a voltage across said dielectric member comprises applying a DC voltage thereacross having a polarity the opposite of said one polarity to eflectively eliminate the built-up space charge.

4. A method according to claim 2; wherein said steps of applying a voltage comprise applying an AC voltage having alternating opposite polarities across said dielectric member and said electrical conductor in synchronization with the advancement of said photoconductive sheet therebetween to apply voltage of one polarity across said dielectric member and said electrical conductor when said dielectric member is in said one part of its cycle and to apply voltage of the opposite polarity across said dielectric member and said electrical conductor when said dielectric member is in said another part of its cycle.

5. Apparatus for charging static electricity upon sheet material in electrophotography comprising: a dielectric member having an electroconductive core position and an earth member urged toward said dielectric member; carrying means for carrying sheet material between and in contact with both said members at a predetermined speed; at least one voltage applying means connected to said electroconductive member and said electroconductive core portion for applying a voltage thereacross effective to electrostatially charge said sheet material while preventing accumulation of space charge in said dielectric member, and said voltage applying means including a second electroconductive member in continuous contact with said dielectric member at a position apart from a nip position of said dielectric member with said earth member, and another voltage applying means connected to said second electroconductive member for applying a voltage having the reverse polarity with respect to said first-mentioned voltage to said second dielectric member at its contacting position.

6. Apparatus according to claim 5, wherein said dielectric member comprises a core roller composed of an electroconductive material and a dielectric surface layer converting said core roller; and means for rotatably mounting said dielectric member.

7. Apparatus according to claim 6, wherein said earth member comprises a roller composed of an electroconductive material, means mounting said earth member for rotation about an axis parallel to the axis of rotation of said dielectric member, means for urging said earth member and said dielectric member toward each other, and wherein said carrying means for carrying said sheet material includes peripheral surface portions of said earth member and said dielectric member.

8. Apparatus according to claim 5, wherein said dielectric member comprises a pair of rotatably mounted electroconductive rollers and a dielectric endless belt driven by said pair of electroconductive rollers, and wherein said carrying means comprises said dielectric member and a mechanical drive means for driving one of said electroconductive rollers.

9. Apparatus according to claim 6, wherein said earth member comprises an electroconductive plate, means urging said electroconductive plate toward said dielectric member, and wherein said carrying means comprises said dielectric member and mechanical means for driving said dielectric roller.

10. Apparatus according to claim 5, wherein said voltage applying means includes a second electroconductive member in continuous contact with said dielectric member at a position apart from a nip position of said dielectric member with said earth member, and another voltage applying means connected to said second electroconductive member for applying a voltage having the reverse polarity with respect to said firstmentioned voltage to said second dielectric member at its contacting position.

11. Apparatus according to claim 10, wherein said second electroconductive member comprises a rotatably mounted electroconductive roller in continuous contact with said dielectric member.

12. Apparatus according to claim 8, wherein said electroconductive member comprises a metallic piece in continuous contact with said dielectric member.

13. In combination: a first electrical conductor; a dielectric covering attached to a peripheral portion of said first electrical conductor; a second electrical conductor; means mounting said first and second electrical conductors for cyclical movement relative to each other wherein during at least one portion of said cyclical movement said first and second electrical conductors are in opposed relationship defining therebetween a sheet passageway; at least one photoconductive sheet; means for advancing a photoconductive sheet through said sheet passageway in contact with both said dielectric covering and said second electrical conductor; voltage applying means for applying a voltage across said dielectric covering and said second conductor when said photoconductive sheet is in said sheet passageway of sufficient magnitude to electrostatically charge said photoconductive accompanied by accumulation of an electrostatic space charge in an exterior portion of said dielectric covering; and additional voltage applying means for applying a voltage across said dielectric covering effective to eliminate electrostatic space charge accumulated in said exterior surface portion of said dielectric covering.

14. A combination according to claim 13; wherein said firstmentioned voltage applying means comprises a DC voltage source electrically connected to said first and second electrical conductors; and wherein said additional voltage applying means comprises another DC voltage source electrically connected across said dielectric covering at a location remote from said sheet passageway.

15. A combination according to claim 13; wherein both said voltage applying means comprise another electrical conductor in continuous contact with said dielectric covering at a location remote from said sheet passageway, and a DC voltage source electrically connected across said another electrical conductor and said second electrical conductor.

16. A combination according to claim 13; wherein both said voltage applying means comprise an AC voltage source having alternating half-cycles of opposite polarity, and means for applying said AC voltage of one polarity across said dielectric covering and said second electrical conductor when a photoconductive sheet is in said sheet passageway and applying said AC voltage of the opposite polarity across said dielectric covering and said second electrical covering when said first and second electrical conductors are in a part of said cyclical movement different from that existing when said photoconductive sheet was in said sheet passageway.

17. A method for applying voltage between a dielectric member and an electroconductive member in electrophotography while carrying a piece of sheet material in a sandwiched condition therebetween, comprising: providing in spacedapart relationship a dielectric member and an electroconductive member; advancing a sheet material between said dielectric member and said electroconductive member; applying at least one voltage between said dielectric member and said electroconductive member for a predetermined time period effective to impart a static charge upon said sheet material while eliminating electrical space charge accumulated in a surface layer of said dielectric member; whereby a continuous electrophotographic operation can be carried out, and the applying step comprising applying a DC voltage across said dielectric member and said electroconductive member to impart a static charge of one polarity on said sheet material, and applying another voltage across said dielectric member having a polarity the reverse of the charge accumulated in said dielectric member.

18. A method for applying voltage between a dielectric member and an electroconductive member in electrophotography while carrying a piece of sheet material in a sandwiched condition therebetween, comprising: providing in spacedapart relationship a dielectric member and an electroconductive member; advancing a sheet material between said dielectric member and said electroconductive member applying at least one voltage between said dielectric member and said electroconductive member for a predetermined time period effective to impart a static charge upon said sheet material while eliminating electrical space charge accumulated in a surface layer of said dielectric member; whereby a continuous electrophotographic operation can be carried out, and the applying step comprising alternately applying a first voltage of one polarity between said dielectric member and said electroconductive member when said sheet material is therebetween to impart static charge and applying, another voltage having a reverse polarity with respect to said first voltage between said dielectric member and said electroconductive member during absence of said sheet material between said members. r v

19. A method for applying voltage between a dielectric member and an electroconductive member in electrophotography while carrying a piece of sheet material in a sandwiched condition therebetween, comprising: providing in spacedapart relationship a dielectric member and an electroconductive member; advancing a sheet material 'betweensaid dielectric member and said electroconductive member applying at least l voltage between said dielectric member and said electroconductive member for a predetermined time period effective to impart a static charge upon said sheet material while eliminating electrical space charge accumulated in a surface layer of said dielectric member; whereby a continuous electrophotographic operation can be carried out, and the applying step comprising alternately applying a first DC voltage and a second DC voltage having a reverse polarity with respect to said first voltage across said dielectric member and said electroconductive member in synchronization with the alternate advancement of said sheet material into and out of position between said members.

20. Apparatus for charging static electricity upon sheet material in electrophotography comprising: a dielectric member having an electroconductive core portion and an earth member urged toward said dielectric member; carrying means for carrying sheet material between and in contact with both said members at a predetermined speed; at least 1 voltage applying means connected to said electroconductive member and said electroconductive core portion for applying a voltage thereacross effective to electrostatially charge said sheet material while preventing accumulation of space charge in said dielectric member said voltage applying means comprising AC voltage applying means for applying an AC voltage between said both members during a period when said sheet material is sandwiched between said both members, and said AC voltage applying means is synchronized for applying a voltage having a reverse polarity between said both members during an absence of said sheet material between said both members, thereby application of voltages is carried out synchronously with the periodical sandwiching of said sheet material between said both members. 

1. A method for charging static electricity upon a sheet material in electrophotography comprising: sandwiching said sheet material between a rotationally driven dielectric roller and an electroconductive member urged toward said rollers; applying a voltage between said dielectric roller and said electroconductive member effective to impart a static charge upon said sheet material while preventing accumulation of space charge in a surface layer of said dielectric roller, and said prevention of space charge accumulation in a surface layer of said dielectric roller is carried out by applying another voltage having a reverse polarity with respect to said first-mentioned voltage to said dielectric roller at a position apart from a nip position between said dielectric roller and said electroconductive roller.
 2. A method for continuously eliminating space charge accumulated in an exterior surface portion of a dielectric member during electrophotography comprising: providing a dielectric member in cyclical motion and an electrical conductor effectively spaced-apart from said dielectric member during at least a portion of said cyclical motion to define therebetween a sheet passageway; advancing a photoconductive sheet through said sheet passageway in contact with both said dielectric member and said electrical conductor; applying a voltage across said dielectric member and said electrical conductor when said photoconductive sheet is in said sheet passageway thrusly defining one part of said cycle of sufficient magnitude to electrostatially charge said photoconductive sheet accompanied by accumulation of an electrostatic space charge in an exterior surface portion of said dielectric member; and then applying a voltage across said dielectric member when same is at another part of its cycle different than said one part effective to eliminate the electrostatic space charge accumulated in said exterior surface portion thereof.
 3. A method according to claim 2, wherein said step of applying a voltage across said dielectric member and said electrical conductor comprises applying a DC voltage thereacross to electrostatially charge said photoconductive sheet accompanied by building up of an electrostatic space charge of one polarity; and wherein said step of applying a voltage across said dielectric member comprises applying a DC voltage thereacross having a polarity the opposite of said one polarity to effectively eliminate the built-up space charge.
 4. A method according to claim 2; wherein said steps of applying a voltage comprise applying an AC voltage having alternating opposite polarities across said dielectric member and said electrical conductor in synchronization with the advancement of said photoconductive sheet therebetween to apply voltage of one polarity across said dielectric member and said electrical conductor when said dielectric member is in said one part of its cycle and to apply voltage of the opposite polarity across said dielectric member and said electrical conductor when said dielectric member is in said another part of its cycle.
 5. Apparatus for charging static electriciTy upon sheet material in electrophotography comprising: a dielectric member having an electroconductive core position and an earth member urged toward said dielectric member; carrying means for carrying sheet material between and in contact with both said members at a predetermined speed; at least one voltage applying means connected to said electroconductive member and said electroconductive core portion for applying a voltage thereacross effective to electrostatially charge said sheet material while preventing accumulation of space charge in said dielectric member, and said voltage applying means including a second electroconductive member in continuous contact with said dielectric member at a position apart from a nip position of said dielectric member with said earth member, and another voltage applying means connected to said second electroconductive member for applying a voltage having the reverse polarity with respect to said first-mentioned voltage to said second dielectric member at its contacting position.
 6. Apparatus according to claim 5, wherein said dielectric member comprises a core roller composed of an electroconductive material and a dielectric surface layer converting said core roller; and means for rotatably mounting said dielectric member.
 7. Apparatus according to claim 6, wherein said earth member comprises a roller composed of an electroconductive material, means mounting said earth member for rotation about an axis parallel to the axis of rotation of said dielectric member, means for urging said earth member and said dielectric member toward each other, and wherein said carrying means for carrying said sheet material includes peripheral surface portions of said earth member and said dielectric member.
 8. Apparatus according to claim 5, wherein said dielectric member comprises a pair of rotatably mounted electroconductive rollers and a dielectric endless belt driven by said pair of electroconductive rollers, and wherein said carrying means comprises said dielectric member and a mechanical drive means for driving one of said electroconductive rollers.
 9. Apparatus according to claim 6, wherein said earth member comprises an electroconductive plate, means urging said electroconductive plate toward said dielectric member, and wherein said carrying means comprises said dielectric member and mechanical means for driving said dielectric roller.
 10. Apparatus according to claim 5, wherein said voltage applying means includes a second electroconductive member in continuous contact with said dielectric member at a position apart from a nip position of said dielectric member with said earth member, and another voltage applying means connected to said second electroconductive member for applying a voltage having the reverse polarity with respect to said first-mentioned voltage to said second dielectric member at its contacting position.
 11. Apparatus according to claim 10, wherein said second electroconductive member comprises a rotatably mounted electroconductive roller in continuous contact with said dielectric member.
 12. Apparatus according to claim 8, wherein said electroconductive member comprises a metallic piece in continuous contact with said dielectric member.
 13. In combination: a first electrical conductor; a dielectric covering attached to a peripheral portion of said first electrical conductor; a second electrical conductor; means mounting said first and second electrical conductors for cyclical movement relative to each other wherein during at least one portion of said cyclical movement said first and second electrical conductors are in opposed relationship defining therebetween a sheet passageway; at least one photoconductive sheet; means for advancing a photoconductive sheet through said sheet passageway in contact with both said dielectric covering and said second electrical conductor; voltage applying means for applying a voltage across said dielectric covering and said second conductor when said photoconductive sheet Is in said sheet passageway of sufficient magnitude to electrostatically charge said photoconductive accompanied by accumulation of an electrostatic space charge in an exterior portion of said dielectric covering; and additional voltage applying means for applying a voltage across said dielectric covering effective to eliminate electrostatic space charge accumulated in said exterior surface portion of said dielectric covering.
 14. A combination according to claim 13; wherein said first-mentioned voltage applying means comprises a DC voltage source electrically connected to said first and second electrical conductors; and wherein said additional voltage applying means comprises another DC voltage source electrically connected across said dielectric covering at a location remote from said sheet passageway.
 15. A combination according to claim 13; wherein both said voltage applying means comprise another electrical conductor in continuous contact with said dielectric covering at a location remote from said sheet passageway, and a DC voltage source electrically connected across said another electrical conductor and said second electrical conductor.
 16. A combination according to claim 13; wherein both said voltage applying means comprise an AC voltage source having alternating half-cycles of opposite polarity, and means for applying said AC voltage of one polarity across said dielectric covering and said second electrical conductor when a photoconductive sheet is in said sheet passageway and applying said AC voltage of the opposite polarity across said dielectric covering and said second electrical covering when said first and second electrical conductors are in a part of said cyclical movement different from that existing when said photoconductive sheet was in said sheet passageway.
 17. A method for applying voltage between a dielectric member and an electroconductive member in electrophotography while carrying a piece of sheet material in a sandwiched condition therebetween, comprising: providing in spaced-apart relationship a dielectric member and an electroconductive member; advancing a sheet material between said dielectric member and said electroconductive member; applying at least one voltage between said dielectric member and said electroconductive member for a predetermined time period effective to impart a static charge upon said sheet material while eliminating electrical space charge accumulated in a surface layer of said dielectric member; whereby a continuous electrophotographic operation can be carried out, and the applying step comprising applying a DC voltage across said dielectric member and said electroconductive member to impart a static charge of one polarity on said sheet material, and applying another voltage across said dielectric member having a polarity the reverse of the charge accumulated in said dielectric member.
 18. A method for applying voltage between a dielectric member and an electroconductive member in electrophotography while carrying a piece of sheet material in a sandwiched condition therebetween, comprising: providing in spaced-apart relationship a dielectric member and an electroconductive member; advancing a sheet material between said dielectric member and said electroconductive member applying at least one voltage between said dielectric member and said electroconductive member for a predetermined time period effective to impart a static charge upon said sheet material while eliminating electrical space charge accumulated in a surface layer of said dielectric member; whereby a continuous electrophotographic operation can be carried out, and the applying step comprising alternately applying a first voltage of one polarity between said dielectric member and said electroconductive member when said sheet material is therebetween to impart static charge and applying another voltage having a reverse polarity with respect to said first voltage between said dielectric member and said electroconductive member during absence of said sheet material between said members.
 19. A method for applying voltage between a dielectric member and an electroconductive member in electrophotography while carrying a piece of sheet material in a sandwiched condition therebetween, comprising: providing in spaced-apart relationship a dielectric member and an electroconductive member; advancing a sheet material between said dielectric member and said electroconductive member applying at least 1 voltage between said dielectric member and said electroconductive member for a predetermined time period effective to impart a static charge upon said sheet material while eliminating electrical space charge accumulated in a surface layer of said dielectric member; whereby a continuous electrophotographic operation can be carried out, and the applying step comprising alternately applying a first DC voltage and a second DC voltage having a reverse polarity with respect to said first voltage across said dielectric member and said electroconductive member in synchronization with the alternate advancement of said sheet material into and out of position between said members.
 20. Apparatus for charging static electricity upon sheet material in electrophotography comprising: a dielectric member having an electroconductive core portion and an earth member urged toward said dielectric member; carrying means for carrying sheet material between and in contact with both said members at a predetermined speed; at least 1 voltage applying means connected to said electroconductive member and said electroconductive core portion for applying a voltage thereacross effective to electrostatially charge said sheet material while preventing accumulation of space charge in said dielectric member said voltage applying means comprising AC voltage applying means for applying an AC voltage between said both members during a period when said sheet material is sandwiched between said both members, and said AC voltage applying means is synchronized for applying a voltage having a reverse polarity between said both members during an absence of said sheet material between said both members, thereby application of voltages is carried out synchronously with the periodical sandwiching of said sheet material between said both members. 